我读到 Dart 不支持函数重载。它支持运算符重载吗?如果是,您能通过一个简单的示例向我展示它是如何完成的吗?还有什么优点等等?
当您在新版本中尝试使用
==
运算符重载时,所选答案不再有效。现在你需要这样做:
class MyClass {
@override
bool operator ==(other) {
// compare this to other
}
}
但这并不安全。
other
未指定为类型,可能会发生意外情况。例如:
void main() {
var a = A(1);
var b = B(1);
var result = a == b;
print(result); //result is true
}
class A {
A(this.index);
final int index;
@override
bool operator ==(other) => other.index == index;
}
class B {
B(this.index);
final int index;
}
所以你可以这样做:
class A {
A(this.index);
final int index;
@override
bool operator ==(covariant A other) => other.index == index;
}
您需要使用
covariant
。因为 Object 重载了 ==
运算符。
测试对象类型:
访问:hash_and_equals
class A {
A(this.index);
final int index;
@override
bool operator ==(other) => other is A && (other.index == index);
@override
int get hashCode => index;
}
Dart 确实支持运算符重载,使用 operator 关键字,后跟要重载的运算符。以下示例重载 MyClass 对象的 == 运算符:
class MyClass {
operator ==(MyClass other) {
// compare this to other
}
}
几乎所有 Darts 内置运算符都可以重载,但有一些值得注意的例外是赋值运算符 = 和引用等价运算符 ===(不再存在)。
至于运算符重载的优点,它允许您重用具有众所周知的语义含义的运算符,例如 == 或 + 来对对象进行操作。例如,如果您有一个重载 + 运算符的 Matrix 类,那么您可以使用语法 m1 + m2 添加两个矩阵,而不是使用更麻烦的 m1.plus(m2)
为了扩展 Lars 的答案,您还可以使用内联函数语法重载运算符。
class MyClass {
operator ==(MyClass o) => id == o.id;
}
学习如何使用运算符重载的一个惊人示例是在 dart 中处理复数的类:
import 'dart:core';
class Complex {
final double real;
final double imaginary;
Complex({this.real = 0, this.imaginary = 0});
Complex.ri(this.real, this.imaginary);
Complex operator +(Complex b) {
return Complex(
real: this.real + b.real, imaginary: this.imaginary + b.imaginary);
}
Complex operator -(Complex b) {
return Complex(
real: this.real - b.real, imaginary: this.imaginary - b.imaginary);
}
Complex operator *(Complex b) {
return Complex(
real: this.real * b.real - this.imaginary * b.imaginary,
imaginary: this.real * b.imaginary + this.imaginary * b.real);
}
Complex operator /(Complex b) {
// https://stackoverflow.com/a/41146661/6846888
var conjugation = b.conjugate();
var denominatorRes = b * conjugation;
// denominator has only real part
var denominator = denominatorRes.real;
var nominator = this * conjugation;
return Complex(
real: nominator.real / denominator,
imaginary: nominator.imaginary / denominator);
}
bool operator ==(b) {
return b.real == this.real && b.imaginary == this.imaginary;
}
@override
String toString() {
return 'Complex(real: ${real}, imaginary: ${imaginary})';
}
}
从 Dart 2.7 版本开始,您可以向现有类添加运算符,例如:
extension Contains on String {
bool operator <<(Pattern other) => contains(other);
bool operator >>(String other) => other.contains(this);
}
使用复数作为示例,我可以实现“Complex * num”,如下所示:
Complex operator *(dynamic b) {
if (b is Complex) {
return Complex( real: ... );
} else if (b is num) {
return Complex.ri(real*b, imaginary*b);
}
}
...